U.S. patent number 4,916,116 [Application Number 07/190,352] was granted by the patent office on 1990-04-10 for method of adding a halogen element into oxide superconducting materials by ion injection.
This patent grant is currently assigned to Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Shunpei Yamazaki.
United States Patent |
4,916,116 |
Yamazaki |
April 10, 1990 |
Method of adding a halogen element into oxide superconducting
materials by ion injection
Abstract
A method of adding a halogen element into oxide superconducting
materials includes the steps of forming a passivation film on an
oxide superconducting material, adding halogen ions into the oxide
superconducting material by ion injection and then applying a heat
treatment in oxygen to the oxide superconducting material.
Inventors: |
Yamazaki; Shunpei (Tokyo,
JP) |
Assignee: |
Semiconductor Energy Laboratory
Co., Ltd. (Atsugi, JP)
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Family
ID: |
26450968 |
Appl.
No.: |
07/190,352 |
Filed: |
May 5, 1988 |
Foreign Application Priority Data
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May 6, 1987 [JP] |
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62-111614 |
Jun 1, 1987 [JP] |
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62-138580 |
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Current U.S.
Class: |
505/461; 427/523;
427/62; 427/63; 505/701; 505/730; 505/785 |
Current CPC
Class: |
C04B
41/0027 (20130101); C23C 14/087 (20130101); C23C
14/58 (20130101); C23C 14/5806 (20130101); C23C
14/5833 (20130101); C23C 14/5853 (20130101); H01L
39/247 (20130101); Y10S 505/701 (20130101); Y10S
505/785 (20130101); Y10S 505/731 (20130101); Y10S
505/73 (20130101) |
Current International
Class: |
C04B
41/00 (20060101); C23C 14/58 (20060101); H01L
39/24 (20060101); C23C 14/08 (20060101); B05D
003/06 () |
Field of
Search: |
;427/62,63,38
;204/192.24 ;252/500 ;505/1,701,785,730 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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58-164278 |
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Sep 1983 |
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JP |
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58-176930 |
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Oct 1983 |
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JP |
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Other References
Kawazaki et al., "Compositional and Structural Analyses for
Optimizing the Preparation Conditions of Superconducting
(La.sub.1-x Sr.sub.x).sub.y CuO.sub.4-8 Films by Sputtering", Jpn.
J. Appl. Phys., vol. 26, No. 4, Apr. (1987) L388-390. .
Politis et al., "Preparation and Superconducting Properties of
La.sub.1-8 Sr.sub.0.2 CuO.sub.4 and YBa.sub.2 Cu.sub.3 O.sub.6.5 ",
Extended Abstract, High Temperature Superconductors, edited by
Gubser et al., Apr. (1987) P141-143. .
Tonouchi et al., "Hall Coefficient of La--Sr--Cu Oxide
Superconducting Compound", Jpn. J. Appl. Phys., vol. 26, No. 4,
Apr., 1987, L519-520..
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Primary Examiner: Morgenstern; Norman
Assistant Examiner: King; Roy V.
Attorney, Agent or Firm: Sixbey, Friedman, Leedom and
Ferguson
Claims
What is claimed is:
1. A method of producing oxide superconducting materials comprising
the steps of:
providing a superconducting oxide;
forming a passivation film on said superconducting oxide;
adding halogen ions into said superconducting oxide after said
forming of the passivation film, wherein said addition of halogen
ions is effected by ion injection;
heat treating said superconducting oxide in oxygen after said
injection of the halogen ions.
2. The method of producing oxide superconducting materials of claim
1, wherein said halogen is fluorine.
3. A method of producing the oxide superconducting materials of
claim 1, wherein the heat treatment is carried out at a temperature
from 250.degree. C. to 500.degree. C.
4. A method of producing the superconducting materials of claim 1
where the oxide superconducting material after the ion injection is
represented by the formula (A.sub.1-x B.sub.x).sub.y Cu.sub.z
O.sub.w X.sub.v where x=0 to 1.0, y=2.0 to 4.0, z=1.0 to 4.0, w=4.0
to 8.0 and v=0 to 3.0, with A being an element selected from group
IIIa of the periodic table, B being an element selected from group
IIa, and X being a halogen element selected from group VIIa.
5. A method of producing the oxide superconducting materials of
claim 1, wherein the thickness of said passivation film is from 100
to 20,000 .ANG..
6. A method of producing the oxide superconducting materials of
claim 1, wherein said passivation film is silicon nitride.
Description
FIELD OF THE INVENTION
This method relates to a method of producing an oxide
superconducting material.
This invention improves the near-surface properties of an oxide
superconducting material for use in a device which uses the surface
of the superconducting material. In addition, for use in a device
such as a superconducting magnet which uses the bulk (interior),
this invention increases the stability, especialy the stability
with respect to oxygen vacancies.
BACKGROUND OF THE INVENTION
Recently, oxide superconducting materials have attracted a great
deal of attention. This activity was started by the development of
a Ba--La--Cu--O-type oxide superconducting material at IBM Zurich.
In addition, yttrium-type oxide superconducting materials are
known. It is clear that these have possible applications in solid
state electronic devices at liquid nitrogen temperature.
Meanwhile, semiconductor materials containing metals, such as
Nb.sub.3 Ge, are also well-known. Attempts have been made to use
these metal superconducting materials in solid state electric
devices such as Josephson elements.
After well over 10 years of research, Josephson elements containing
these metal superconducting materials have almost reached the stage
of practical application. However, these superconducting materials
have a T.sub.co (temperature at which the electrical resistance
becomes zero) at 23.degree. K., which is very low, and so cannot be
used without using liquid helium, which is not sufficiently
practical.
Meanwhile, since these metal superconducting materials are made
entirely of metals, their composition is uniform, both on the
surface and in the interior (bulk).
However, when the characteristics of oxide superconducting
materials, which have recently attracted attention, are
investigated, it is found that their near-surface characteristics
(to a depth of about 200 .ANG.) are inferior (less reliable)
compared to those in the interior in many cases.
Investigation of the properties of this material has shown that it
has a T.sub.co of 90.degree. K. to 100.degree. K., and in addition,
the electrical conductivity varies in the range of 150.degree. K.
to 270.degree. K.
The cause of this is judged to be that the oxygen in an oxide
superconducting material escapes into the air easily when it is
near the surface. When the material is heated to 250.degree. C. to
500.degree. C., the oxygen escapes easily even from the interior,
causing many oxygen vacancies to be produced. The term of "vacancy"
is used to mean an opening where an atom is missing in the regular
arrangement of atoms. Whether this oxygen is present in its full
amount or insufficient is a critical factor in determining whether
the material can be made superconducting or is simply an ordinary
electrically conducting material.
This invention enables an oxide superconducting material to be
superconducting both in the interior and near the surface, with an
optimum density of oxygen vacancies, heat resistance and process
resistance which means that the oxide superconducting material can
be kept stable even in a vaccum.
Thus, by adding a halogen element to the vacancy, the oxide
superconducting material becomes mechanically stronger, so that
even if it is made into a thin film, it can be given a T.sub.co of
90.degree. K. or higher. In particular, when it is formed by
sputtering, in general it will be formed densely so that it is hard
to produce a vacancy, but by adding fluorine to whatever vacancies
do form before or at the same time as the thin films is formed, the
film that is obtained is dense, heat-resistant and
process-resistant.
SUMMARY OF THE INVENTION
The object of this invention is to provide a method to overcome the
problems as mentioned above, in which a halogen element is added to
the oxide superconducting material to fill part or all of the
oxygen vacancies.
Another object of this invention is to provide a method in which,
when the oxide superconducting material film is formed, sputtering
is used, so that halogen elements added during the process fill
part or all of the oxygen vacancies and cancel those vacancies.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(A) to 1(E) show a production method of the present
invention and a corresponding oxygen concentration
distribution.
FIG. 2 shows a diagram of a sputtering system used in the present
invention .
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In one embodiment of the present invention, a halogen element is
added to the oxide superconducting material to fill part or all of
the oxygen vacancies. In particular, addition of a halogen element
such as fluorine to a superconducting that has a certain number of
oxygen vacancies and a maximum T.sub.co fills part or all of the
vacancies, thus stabilizing the pervskeit molecular structure. As a
result it become heat-resistant and process-resistant. This method
is particularly effective for a thin film material that has a large
surface area.
At the same time, this embodiment involves the simultaneous
formation of a coating on the surface of the oxide superconducting
material. Before or after this step, a halogen element, generally,
fluorine, is added by a method such as ion injection or heat
oxidation; then the whole superconducting material is heat-treated
to set the added fluorine into the proper arrangement of atoms.
In addition, a more complete blocking layer is formed by
heat-treating the coating and oxidated to produce an insulating
film in the case of metal and semiconductor.
By causing solid-solid diffusion of the oxygen in the coating, that
is oxygen diffusion from the solid coating into the oxide
superconducting material which is also a solid, the oxygen density
in the vicinity of the surface, generally to a depth of about 200
.ANG., can be adjusted to an appropriate value.
Coatings used for this purpose can be insulators such as silicon
niride, aluminum nitride, aluminum oxide, tantalum oxide or
tiatnium oxide.
A metal or semiconductor that becomes insulating after oxidation
can also be used for this coating. Suitable metals include
aluminum, titanium, tantalum, copper, barium and yttrium; while
suitable semiconductors include silicon and germanium. These are
oxidized to form, for example, aluminum oxide, titanium oxide,
tantalum oxide, copper oxide, barium oxide or yttrium oxide.
Silicon is oxidized to silicon oxide, germanium to germanium
oxide.
The method of this invention is effective whether the oxide
superconducting material is formed into a tablet or this film.
Methods of forming a thin film include screen printing, sputtering,
MBE (molecular beam epitaxy), CVD (chemical vapor deposition
reaction) and photo CVD.
In another embodiment of this invention, when the target is
sputtered and superconducting material flies off and forms on a
surface to be coated, the gas which strikes this target has both
oxygen and a halogen element added to the inert gas at the same
time. This halogen-containing gas becomes a plasma, and the halogen
element, for example fluorine, is added to the superconducting
oxide material that is formed on the surface. After that the whole
thing is heat-treated, and the added fluorine is embedded at the
locations where vacancies were produced.
An oxide superconducting material has a molecular structure, for
example, represented by (A.sub.1-x B.sub.x).sub.y Cu.sub.z O.sub.w,
where x=0 to 1, y=2.0 to 4.0 and preferably 2.5 to 3.5, z=1.0 to
4.0 and preferably 1.5 to 3.5, and w=4.0 to 10.0 and preferably 6.0
to 8.0, and A is at least one element selected from Group IIIa of
the Periodeic Table, for example Y(yttrium), Gd(gadolinium),
Yb(ytterbium), Eu(Europium), Tb(Terbium), Dy(Dysprosium),
Ho(Holmium), Er(Erbium), Tm(Thulium), Lu(Lutetium), Sc(Scandium) or
other lanthanides, and B is an element selected from Group IIIa of
the Periodic Table, for example, Ba(barium) or Sr(strontium),
Ca(calcium), Mg(Magnesium), Be(Beryllium).
Fluorine fills vacancies in the material most easily since it has
the smallest atomic radius.
As a specific example, (YBa.sub.2)Cu.sub.3 O.sub.6-8 is used for
the first embodiment. In addition, another lanthanide elements and
actinide elements can be used.
As a specific example for the second embodiment,
(YBa.sub.2)Cu.sub.3).sub.6-8 X.sub.2-0.01 was used. In addition to
the elements listed above, lanthanide elements and actinide
elements can be used as A.
In this invention, a halogen element such as fluorine is added to
the said oxide superconducting material in a concentration of
1/100% to 1/200% compared to the concentration of vacancies in the
case in which none is added, to add heat-resistance and
process-resistance.
In this invention, a halogen element such as fluorine is added to
the oxide superconducting material in a concentration 1/100 to
1/200% that of the vacancies. This produces a superconducting that
is heat-resistant and process-resistant. In addition, to prevent
the escape of more oxygen from the superconducting material, a
deterioration prevention film or passivation film is also formed on
the surface.
If the insulation coating has a thickness that permits the passage
of a 5 to 50 .ANG. tunnel current, another superconducting material
can be laid on top of this insulating coating to form a Josephson
element.
Also, the passivation film can be made in a thickness from 100 to
20,000 .ANG. so that it also serves as a deterioration prevention
coating.
In this invention, after a halogen element such as fluorine is
added to the oxide superconducting material, the superconducting
material together with the halogen is heat-treated in an inert gas,
air or oxygen at 250.degree. C. to 500.degree. C., for example at
500.degree. C., for 2 to 50 hours, for example for 5 hours. This
heat treatment causes the fluorine that was added by ion injection,
and, if any, the oxygen that was added in addition to the fluorine,
to be set in a suitable atomic arrangement to form a stable
superconducting surface. The reason for using a relatively low
temperature is that escape of oxygen from the superconducting
material and its replacement by fluorine set in the vacancies
occurs easily at such relatively low temperature.
As a result, the surface oxygen concentration can be held at a
suitable value when the super conducting material is maintained at
liquid nitrogen temperature; that is to say, a passivation film can
be produced.
This process has solved the problem of unreliability of oxide
superconducting materials, with the superconductivity near the
surface disappearing suddenly due to an unexplained cause. The
surface remains stably superconducting for a long time.
By adding fluorine uniformly through the material, including the
interior, the superconductivity which it has previously acquired
becomes fixed. The T.sub.co of the superconducting material becomes
higher and it remains usable at a higher current density, which are
important properties. Until now even though a high T.sub.co was
obtainned and a high current density was used, if the
superconducting material was left in a vacuum and a large current
allowed to continuously flow through it, it deteriorated. By adding
a halogen element as in this invention in sufficient concentration
to cancel the oxygen vacancies (1/100 to 1/200% of the
concentration of the vacancies), T.sub.co is stabilized. In
addition, the electric current density can be raised to 1500
A/cm.sup.2 or more, up to three times that when halogen is not
added.
As a result, a device whieh uses the surface, such as a Josephson
element, gives stable and reliable operation for a long time.
This invention will now be explained with reference to the
drawings.
EXAMPLE 1
FIG. 1 shows the manufacturing process of an example of this
invention and the corresponding oxygen concentration
distribution.
FIG. 1(A) shows YBa.sub.2 Cu.sub.3 O.sub.6-8 as one example of an
oxide superconducting material. The copper amount can be 3 or less.
Such a superconducting material can have a monocrystalline or
polycrystalline structure on a tablet or thin film; it is the
starting material (FIG. 1(A) (1)).
When this material is held in a vacuum device and the air
evacuated, the oxygen escapes from near the surface (1'), causing
deterioration of the electrical characteristics to a depth of about
200 .ANG..
The oxygen concentration corresponding to FIG. 1(A) is shown in
FIG. 1(D). In this figure, region (1) has normal oxygen
concentration. Region (1') is short of oxygen. This depth depends
on the type, structure and density of the superconducting material,
but it varies from 50 .ANG. To 1000 .ANG.; in general it is about
200 .ANG..
On top of the material, a silicon nitride layer 5 .ANG. to 50 .ANG.
thick, for example 20 .ANG. thick, is formed by photo CVD wherein
ultraviolet radiation or laser light is used, so that a reactive
gas is excited by the radiation, causing a coating to be formed on
the surface. In addition, ion injection is carried out. The
acceleration voltage is weak, 10 to 30 KV; ions are added in such a
manner that the oxygen concentration becomes fixed. Then the
superconducting material is heat-treated at 350.degree. C. for 2
hours. In this invention the voltage used to accelerate the
fluorine, which is used as the halogen element, can be varied from
10 to 500 KV, so that on the average the fluorine is added in a
concentration 1/100 to 1/200% that of the vacancies, for example
3.times.10.sup.21 cm.sup.-3.
The superconducting material is then heat-treated in oxygen at
250.degree. C. to 500.degree. C., for example at 350.degree. C.,
for about 30 minutes. As a result of this heat treatment, fluorine
penetrates into oxygen vacancies in the interior, as shown in FIG.
1(E), making it hard for the pervskeit structure of this oxide
superconductor material to deteriorate.
The sample produced in a test of this example was removed from the
heated condition and then stored in vacuum at 350.degree. C. for 5
hours. The superconducting material with fluorine added had no
oxygen deficiency and formed a device of high reliability.
An oxide superconducting material produced according to the method
of this invention has micro-scale surface depressions on a scale
observable under an electron microscope. These depressions have
gaps in its interior. This causes the surface to appear very large.
To make this surface become passive, fluorine, which is the most
electrically negative halogen, is coated in a single layer or in
depression. This is very effective in increasing the heat
resistance. In addition, halogen can be added to fill the surface
and interior oxygen vacancies. This procedure is very effective in
simplifying the superconducting material production process.
In a test of this process, the oxide superconducting material with
fluorine added was left in a vacuum at 300.degree. C. for 5 hours.
A coatng with fluorine added according to the method of this
invention kept stable superconductivity with a T.sub.co of
79.degree. K. Under the same conditions, however, an oxide
superconducting material to which fluorine had not been added lost
its superconductivity completely.
In describing this invention, the term "oxide superconducting
material" has been used. It is clear in the technological concept
of this invention that this material can have either a
menocrystalline or polycrystalline structure.
In the example of this invention that was described, fluorine was
used as the halogen element. However, iodine or bromine could also
be added.
In the previous example, after the coating is formed, oxygen is
injected into the superconducting material by ion injection. It is
also effective to first inject oxygen by ion injection into the
surface and immediate vicinity of the surface of the
superconducting material, then form the coating, then use heat
treatment to cause the added oxygen to assume the correct atomic
arrangement to make the material superconducting.
In describing the previous example, fluorine is added to a
previously formed material. However since the usual way of
producing this superconducting material uses finely-grained yttrium
oxide, barium carbonate and copper oxide which are repeatedly
blended and fired, then formed into a tablet, and if a thin film is
to be formed, this table is used as a target for sputtering to form
the thin film on a mold, any or all of YF.sub.3, YbF.sub.3,
TbF.sub.3 and LaF.sub.3 can be added to the original materials to
be blended as a means of adding fluorine. The corresponding
chlorides or bromides could be used in place of these
fluorides.
However, since the basic concept of this invention is to first
produce oxygen vacancies in the superconducting material at
whatever temperature is required to produce them, then to which a
halogen element is added to fill them, it is desirable to add the
halogen after first forming the oxide suprerconductor material so
as to increase T.sub.co.
EXAMPLE 2
FIG. 2 shows an outline of a sputtering device that is used to
produce the superconducting material of this invention.
In FIG. 2, there are a target (1), a reaction chamber (4), a doping
system (10) and an exhaust system (30).
Argon (5), oxygen (6) and a halogen-containing gas (7) are
introduced into the doping system. Here nitrogen fluoride
(NF.sub.3) is used for the halogen-containing gas (7). The exhaust
system (30) has a turbo pump (8), pressure adjusting valve (9), and
rotary pump (11). The substrate (2) is laid on a holder (3), which
also surves as a heater, and is heated from room temperature up to
a maximum of 900.degree. C.
While the film is being formed the temperature is kept between
400.degree. C. and 900.degree. C., for example at 750.degree. C.
The target (1) and substrate (2) are 2 cm to 15 cm from the surface
to be coated.
The target (1) is made of an oxide superconducting material which
has the formula (A.sub.1-x B.sub.x).sub.y Cu.sub.z O.sub.w X.sub.v,
where x is 0 to 1.0, y is 2.0 to 4.0, z is 1.0 to 4.0, w is 4.0 to
8.0 and v is 0 to 3.0, and is pressed. The rear surface of this
so-called target (12) has a packing plate (13), a magnet (14), a
cooling water inlet (15), a cooling water outlet (16) and a shield
plate (17). These are electrically isolated from the main body of
the sputering device by a teflon insulater (18). A large negative
voltage with respect to this target (1) is applied to the electric
current input terminal (20).
When DC (direct current) sputtering is used, negative voltage is
applied to this target and the substrate is grounded.
When AC sputtering is used, the substrate is electrically
floated.
EXPERIMENT 1
YBa.sub.2 Cu.sub.3 O.sub.6 to 8 is used as the target (12). The
target and the substrate are 10 cm apart. The argon partial
pressure is 4.times.10.sup.-1 Pa, the oxygen partial pressure
5.times.10.sup.-3 Pa and the NF.sub.3 partial pressure
8.times.10.sup.-4 Pa. The DC sputtering output is 500 W, 1 KW. This
target has a diameter of 20 cm. The substrate (2) is in a holder
(3) which is heated to 750.degree. C. and rotated so that it
becomes uniform in thickness. It is then slowly cooled to a
temperature between 250.degree. C. and 500.degree. C. at which it
is held for eight hours to deform the crystal structure in the
film. In this experiment we were able to produce an oxide
superconducting material having a T.sub.co of 215.degree. K.
When NF.sub.3 was not introduced at all in this experiment,
T.sub.co was only 83.degree. K. From this we conclude that when
fluorine, which is a halogen element, is added to the film, and
then the film is slowly heated (heat insulated), fluorine is added
at positions where there would otherwise be oxygen vacancies, thus
stabilizing the structure and greatly increasing T.sub.co. If the
heat annealing is not done, there are cases in which
superconductivity is not observed at all; also, T.sub.co exceeds
700.degree. K.
EXPERIMENT 2
Y.sub.0.5 Yb.sub.0.5 Ba.sub.2 Cu.sub.3 O.sub.6 to 8 X.sub.2 to 0.01
was used as the target. In this case, fluorine had already been
added as X. The argon pressure was 4.times.10.sup.-1 Pa; neither
oxygen nor NF.sub.3 was added. The film produced (thickness 2
micrometers) was heat annealed at 300.degree. C. for 5 hours in
air. As a result, the superconducting material formed on the
surface had 600 .ANG./minute. T.sub.co of 143.degree. K. was
obtained.
In the present invention, another type of ceramic compositions can
be used for the superconducting ceramic material.
Specifically, the superconducting ceramic material for use in
accordance with the present invention may be prepared consistent
with the stoichiometric formula (A.sub.1-x B.sub.x).sub.y Cu.sub.z
O.sub.w, where A is one or more elements of Group IIIa of the
periodic Table, e.g. the rare earth elements; B is one or more
elements of Group IIa of the Periodic Table, e.g. the alkaline
earth metals including beryllium and magnesium, and X=0 to 1; y=2.0
to 4.0, preferably 2.5 to 3.5; z=1.0 to 4.0, preferably 1.5 to 3.5;
z=1.0 to 4.0, preferably 1.5 to 3.5; and w=4.0 to 10.0, preferably
6.0 to 8.0. Also, superconducting ceramics for use in accordance
with the present invention may be prepared consistent with the
stoichiometric formula (A.sub.1-x B.sub.x).sub.y Cu.sub.z O.sub.w,
where A is one ore more elements of Group Vb of the Periodic Table
such as Bi, Sb and As; B is one or more elements of Group IIa of
the of the Periodic Table, e.g. the alkaline earth metals including
beryllium and magnesium, and x=0 to 1; y=2.0 to 4.0, preferably 2.5
to 3.5; z=1.0 to 4.0, preferably 1.5 to 3.5; and w=4.0 to 10.0,
preferably 6.0 to 8.0. One example of the former formula is
YBa.sub.2 Cu.sub.3 O.sub.x (x=6 to 8), and examples of
BiSrCaCu.sub.2 O.sub.x and Bi.sub.4 Sr.sub.3 Ca.sub.3 Cu.sub.4
O.sub.x. In addition, the composition Bi.sub.4 (Sr.sub.y
Ca.sub.2)Cu.sub.4 O.sub.x is possible for such purposes and its Tc
is 40 to 60 when the value of y is about 1.5. The Tc onset and Tco
of the composition. Bi.sub.4 Sr.sub.4 Ca.sub.2 Cu.sub.4 O.sub.x are
110.degree. K. and 79.degree. K., respectively. The value of x in
the above formula is estimated to be 6 to 10, for example about
8.1.
The stoichiometric formulae mentioned above can be determined for
example by X-ray diffraction.
* * * * *